1. Field of the Invention
This invention relates to an air fuel ratio sensor which is installed in an exhaust system of an automotive internal combustion engine for controlling an air fuel ratio of the internal combustion engine.
2. Description of the Related Art
Conventionally, an air fuel ratio sensor is installed in an exhaust system of an automotive internal combustion engine to control an air fuel ratio. As shown in FIG. 1, the air fuel ratio sensor 9 is typically composed of a solid electrolyte body 131 having oxygen ion conductivity, a measurement electrode 111 disposed on the front surface of the electrolyte body 131, a reference electrode 112 disposed on the back surface of the electrolyte body 131, and a reference gas chamber 10 in which the reference electrode 112 is disposed. The air fuel ratio sensor 9 further has a heater 15 including a heating resistor 150.
The air fuel ratio sensor 9 is the so-called limiting current type oxygen sensor. That is, when a voltage is applied across the measurement electrode 111 and the reference electrode 112, this type of the air fuel ratio sensor provides output current characteristics with a flat portion with respect to the voltage. The flat portion represents a limiting current value corresponding to an oxygen gas concentration contained in measurement gas. Then, the air fuel ratio is detected based on the oxygen gas concentration. Therefore, it is desired for the air fuel ratio sensor that the output current characteristics clearly have the flat portion with respect to the voltage in a wide range to precisely detect the limiting current value.
In the conventional structure of the air fuel ratio sensor, however, when the air fuel ratio is shifted to a rich side so that the oxygen gas concentration in exhaust gas (measurement gas) decreases, the output current of the air fuel ratio sensor readily changes in response to the change in voltage not to have the flat portion corresponding to the limiting current value. As a result, the oxygen gas concentration and the air fuel ratio may not be precisely detected.
In more detail, when the oxygen gas concentration in the measurement gas is decreased to some extent, the output current flowing between the measurement electrode and the reference electrode is increased so largely that the oxygen gas introduced into the reference gas chamber becomes short. At that time, the output current cannot clearly have the flat portion with respect to the voltage.